Method and apparatus for measuring chemical concentration in a fluid

ABSTRACT

A chemical concentration detecting system for determining the relative concentrations of a multiple component chemical solution. The multi-component solution is preferably comprised of an antimicrobial chemical and a base fluid that acts as a diluent for the antimicrobial chemical, or as a vehicle or carrier for the antimicrobial chemical. A capacitor is exposed to the decontamination solution, wherein the decontamination solution acts as the dielectric between the plates of the capacitor. Permittivity of the dielectric is affected by the relative concentrations of the components, and thus a measurement of the capacitance is used to determine the relative concentration levels of the components in the solution.

FIELD OF THE INVENTION

[0001] The present invention relates to determining chemicalconcentrations in a fluid, and more particularly to a method andapparatus for measuring the concentration of a chemical in a fluidcomprised of multiple chemical components.

BACKGROUND OF THE INVENTION

[0002] The degree of polarity of a molecule is expressed in terms of a“dipole moment.” Molecules, such as water, that exhibit a separation ofcharge within the molecule, have non-zero dipole moments. If theseparated charges are equal in magnitude but opposite in sign, themagnitude of the dipole moment is equal to the product of the value ofone of the separated charges and the distance of separation between thecharges. The dipole moment is a vector that points from the negativelycharged side of the molecule to the positively charged side of themolecule. The dipole moment depends on three factors, namely, (1)polarity of the molecule, (2) the magnitude of the separated charge, and(3) the geometry of the molecule. It is known that different moleculeswill have different dipole moments. For instance, molecules ofantimicrobial chemicals, such as ozone (O₃), and hydrogen peroxide(H₂O₂), have different dipole moments than molecules of water (H₂O).

[0003] The present invention uses differences in the dipole moments ofdifferent molecules as a means for determining chemical concentrationsin a multi-component fluid.

SUMMARY OF THE INVENTION

[0004] In accordance with the present invention, there is provided achemical concentration detecting system for determining a concentrationof a first component in a multi-component solution, comprising: (1) acapacitor having first and second plates exposed to the solution, saidsolution being a dielectric therebetween; and (2) processing means fordetermining a change in an electrical property of the capacitor, saidchange in the electrical property varying according to the concentrationof the first component in the solution.

[0005] In accordance with another aspect of the present invention, thereis provided a method for determining a concentration of a firstcomponent in a multi-component chemical solution, comprising the stepsof: (1) exposing a capacitor, having first and second parallel plates,to the solution, said solution comprising a dielectric therebetween; and(2) determining a change in an electrical property of the capacitor,said change in the electrical property varying according to theconcentration of the first component in the solution.

[0006] An advantage of the present invention is the provision of aconcentration measuring system that uses a fluid as the dielectric of acapacitor.

[0007] Another advantage of the present invention is the provision of aconcentration measuring system that will measure the concentration of awide variety of chemicals, including antimicrobial chemicals.

[0008] Still another advantage of the present invention is the provisionof a concentration measuring system that provides an accuratemeasurement of chemical concentrations in a fluid.

[0009] Yet another advantage of the present invention is the provisionof a concentration measuring system that is simple and inexpensive tomanufacture.

[0010] These and other advantages will become apparent from thefollowing description of a preferred embodiment taken together with theaccompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention may take physical form in certain parts andarrangement of parts, a preferred embodiment of which will be describedin detail in the specification and illustrated in the accompanyingdrawings which form a part hereof, and wherein:

[0012]FIG. 1 is a block diagram of a chemical concentration detectingsystem, according to a preferred embodiment of the present invention;

[0013]FIG. 2 is a schematic diagram illustrating a sensor circuit,according to a preferred embodiment of the present invention; and

[0014]FIG. 3 is a schematic diagram illustrating a sensor circuit,according to an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0015] While the present invention is described herein with reference todetermination of a fluid concentration in a multi-componentdecontamination solution, it should be appreciated that the presentinvention finds utility in measuring a fluid concentration in othertypes of multi-component solutions, including solutions having multiplecomponents wherein the dipole moments of the components differ.

[0016] Referring now to the drawings wherein the showings are for thepurposes of illustrating a preferred embodiment of the invention onlyand not for purposes of limiting same, FIG. 1 shows a chemicalconcentration detecting system 10 according to a preferred embodiment ofthe present invention. Detecting system 10 is generally comprised of asensor circuit 20, a processing unit 50 and an output unit 60.

[0017] Sensor circuit 20 uses a capacitor to sense concentration ofchemicals in a multi-component fluid, as will be described in detailbelow. In this regard, it should be appreciated that the dielectricconstant of a capacitor is dependent on electronic “polarizability.”Polarization is the ability of molecules to form a dipole under anelectric field or the ability of the electric field to line up or rotatean inherent dipole, such as water molecules.

[0018] In a preferred embodiment, processing unit 50 may take the formof a microcomputer or microcontroller, including a memory 52 for datastorage. Processing unit 50 may also be used to control the operation ofother system elements, such as flow controls for controlling fluid flowof components of a decontamination solution. Output unit 60 providesinformation in an audible and/or visual form. Accordingly, output unit60 may take the form of an audio speaker and/or visual display unit.

[0019] Referring now to FIG. 2, there is shown a detailed schematic ofsensing circuit 20. In the preferred embodiment, sensor circuit 20 takesthe form of a “bridge circuit.” As is well known to those skilled in theart, bridge circuits are used to determine the value of an unknownimpedance in terms of other impedances of known value. Highly accuratemeasurements are possible because a null condition is used to determinethe unknown impedance. In the preferred embodiment, the bridge circuitis used to determine a capacitance value indicative of the concentrationof chemicals in a multi-component fluid. In the embodiment shown in FIG.2, sensing circuit 20 is generally comprised of a voltage source 22, anull detector 30, an electronic potentiometer 40, a capacitor C₁ ofknown capacitance, and a capacitor C_(x). Capacitor C₁ is conventionalcapacitor located outside vessel, tank or chamber 100.

[0020] Capacitor C_(x) is directly exposed to a decontamination solutionhaving multiple chemical components. In this regard, capacitor C_(x) islocated in a vessel, tank or chamber 100, wherein the decontaminationsolution fills the gap between the conducting plates of capacitor C_(x),thereby acting as the insulator or “dielectric” of capacitor C_(x).Sensor circuit 20 provides data indicative of a capacitance C_(x),corresponding to a chemical concentration. In this regard, capacitanceC_(x) will vary in accordance with the concentration of components inthe multi-component fluid.

[0021] In a preferred embodiment, capacitor C_(x) is a parallel platecapacitor. However, it should be appreciated that capacitor C_(x) couldbe constructed in a different form. For example, C_(x) could be acylindrical or spherical capacitor. If a spherical capacitor is used ascapacitor C_(x), holes must be placed in the outer shell of thecapacitor such that the chemical components can enter and exit thecapacitor.

[0022] Electronic potentiometer 40 functions in the same manner as amechanical potentiometer. In this regard, electronic potentiometer 40 isa three terminal device. Between two of the terminals is a resistiveelement. The third terminal known as the “wiper” is connected to variouspoints along the resistive element. The wiper is digitally controlled byprocessing unit 50 (see FIG. 1). The wiper divides the resistive elementinto two resistors R_(BC) and R_(AC). Electronic potentiometer 40 maytake the form of a digitally programmable potentiometer (DPP™) availablefrom Catalyst Semiconductor, Inc. of Sunnyvale, Calif.

[0023] In a preferred embodiment, voltage source 22 provides an ACvoltage signal, such as a sinusoidal or pulse waveform. Null detector 30is a device for detecting a null condition (i.e., a short circuit), suchas a galvanometer, a voltmeter, a frequency-selective amplifier, and thelike.

[0024] Operation of sensor circuit 20 will now be described in detail.The elements of the bridge circuit are connected between junctions AC,BC, AD, and BD. Electronic potentiometer 40 is operated by processingunit 50 to vary the resistances R_(BC) and R_(AC) until the potentialdifference between junctions A and B (V_(AB)) is zero. When thissituation exists, the bridge is said to be balanced or is “!nulled.” Thefollowing relationships then hold for voltages in the main branches:

V_(AC)=V_(BC), and V_(AD)=V_(BD),

[0025] where V_(AC) is the voltage between junctions A and C, V_(BC) isthe voltage between junctions B and C, V_(AD) is the voltage betweenjunctions A and D, and V_(BD) is the voltage between junctions B and D.Accordingly,

V _(AD) /V _(AC) =V _(BD) /V _(BC)

V _(AD) =V _(BD)/(V _(AC) /V _(BC))

[0026] The capacitance of capacitor C_(x) is connected between junctionsA and D with a known capacitance of capacitor C₁ between junctions B andD. Electronic potentiometer 40, connected from junction A to junction Cto junction B, is adjusted by processing unit 50 to vary the voltagesV_(AC) and V_(BC).

[0027] When a null is detected by null detector 30, current I₁ flowsfrom junction C to junction A to junction D, and a current I₂ flows fromjunction C to junction B to junction D. The voltage V_(AC) acrossjunctions A to C, and the voltage V_(BC) across junctions B to C are:

V_(AC)=I₁R_(AC) and V_(BC)=I₂R_(BC).

[0028] The voltage across a capacitor with capacitance C, current 1, andfrequency is: $V = \frac{1}{2\pi \quad {fC}}$

[0029] Therefore, the voltages V_(AD) and V_(BD) may be expressed as:$V_{AD} = \frac{I_{1}}{2\pi \quad {fC}_{x}}$$V_{BD} = \frac{I_{2}}{2\pi \quad {fC}_{1}}$

[0030] As discussed above, V_(AD)=V_(BD)/(V_(AC)/V_(BC)),V_(AC)=I₁R_(AC), and V_(BC)=I₂R_(BC). Therefore,$C_{x} = {{C_{1}\left( \frac{R_{BC}}{R_{A\quad C}} \right)}.}$

[0031] In view of the forgoing relationship, when a null condition isdetected, the resistance values for R_(BC) and R_(AC), along with theknown capacitance value of capacitor C₁, can be used to determineunknown value of capacitance for capacitor C_(X).

[0032] Chemical concentration detecting system 10 utilizes differencesin dipole moments of different molecules to determine the relativeconcentration of a chemical in a solution. As discussed above, thedecontamination solution fills the gap between the conducting plates ofcapacitor C_(x), thereby acting as the dielectric of capacitor C_(x). Byconfiguring capacitor C_(x) as an element of a bridge circuit, a measureof resistance values R_(AC) and R_(BC), when the bridge is balanced ornulled, can be used to determine the capacitance of capacitor C_(x). Thecapacitance of capacitor C_(x) is indicative of the relativeconcentrations of the chemical components in the decontaminationsolution, since the permittivity of the respective dielectric isaffected by the relative concentrations of the chemical components ofthe decontamination solution.

[0033] It is well known that for a parallel plate capacitorC=(kε₀)(A/d)=(ε)(A/d), where C is capacitance, k is the dielectricconstant, ε₀ is the permittivity of free space (8.85×10⁻² F/m), ε is thepermittivity (Farads/meter) of the capacitor dielectric, A is the areaof the capacitor plates (m²), and d is the separation in meters betweenthe capacitor plates. As ε increases, the capacitance C will increase.Where the capacitor is a parallel plate capacitor with circular platesof diameter D, C=(πD²ε)/(4d).

[0034] It will be appreciated that the dielectric constant k of thecapacitor can be determined according to the following expression:${k = \frac{4{dC}}{\pi \quad D^{2}ɛ_{0}}},$

[0035] where the value of capacitance, C, is determined as discussedabove. The dielectric constant of the capacitor can also be determinedby determining the capacitance with the dielectric in place between theconducting plates (C_(d)), and then determine the capacitance withoutthe dielectric in place (C_(o)). The ratio of the two capacitancesequals the dielectric constant, $k = {\frac{C_{d}}{C_{0}}.}$

[0036] The response of a capacitor is influenced by the characteristics(e.g., frequency) of the AC waveform applied thereto. In this regard,capacitive reactance (X_(c)) is a function of frequency. Capacitivereactance is the opposition offered to the flow of alternating currentby pure capacitance, and is expressed in ohms (X_(c)=I/(2πfC)).Accordingly, frequency of the waveform generated by voltage source 22influences the response of capacitors. Thus, the frequency selected forvoltage source 22 should preferably be a frequency that will provide agenerally linear response for capacitance as the concentration of achemical component is varied. This will facilitate the use ofinterpolation and extrapolation of capacitance values, as will bediscussed further below. If a suitable linear response is not obtained,then an expanded set of data points should be stored in memory 52.

[0037] It should be appreciated that while a preferred embodiment of thepresent invention includes a sensor circuit 20 in the form of a bridgecircuit, other types of circuits and techniques (including other typesof bridge circuits, and capacitance meters) known to those skilled inthe art, may be suitably used to measure capacitance. For example, FIG.3 illustrates an alternative sensor circuit 20′. Sensor circuit 20′ isan LC resonant circuit, having a variable capacitor C_(A) locatedoutside vessel 100, and a capacitor C_(B) directly exposed to adecontamination solution having multiple chemical components. In thisregard, capacitor C_(B) is located in vessel 100, wherein thedecontamination solution fills the gap between the conducting plates ofcapacitor C_(B), thereby acting as the insulator or “dielectric” ofcapacitor C_(B). Since the resonance frequencyω₀=[L(C_(A)+C_(B))]^(−1/2), the unknown capacitance of capacitor C_(B)can be determined.

[0038] With reference to FIGS. 1 and 2, operation of chemicalconcentmation detecting system 10, according to a preferred embodiment,will now be described in detail. As a preliminary step, processing unit50 stores in memory 52 a set of data comprising values of thecapacitance of capacitor C_(X) for a plurality of relativeconcentrations of a multi-component decontamination solution. This setof data may be determined by exposing capacitor C_(x) of system 10 toseveral different combinations of relative concentrations of themulti-component decontamination solution, and recording thecorresponding measured capacitance C_(x). For example, processing unit50 may store values of the capacitance of capacitor C_(x) that aredetermined for a plurality of relative concentrations of amulti-component decontamination solution comprised of only twocomponents. As the relative concentrations of the first and secondcomponents are varied, the corresponding capacitance of capacitor C_(x)is determined, and stored in memory 52. For instance, capacitance ofcapacitor C_(x) may be determined for various concentrations of a firstcomponent and a second component (at a fixed volume of thedecontamination solution) including, but not limited to:

[0039] 0% first component and 100% second component,

[0040] 25% first component and 75% second component,

[0041] 50% first component and 50% second component,

[0042] 75% first component and 25% second component, and

[0043] 100% first component and 0% second component.

[0044] After the set of data is stored in memory 52, measurement ofconcentrations of a multi-component decontamination solution cancommence. Capacitor C_(x) is exposed to a multi-componentdecontamination solution that is being monitored. As indicated above,capacitor C_(x) may be located in a vessel, tank or chamber 100 filledwith the multi-component solution. A determination of R_(AC) and R_(BC)when the bridge is nulled is then used to determine a value for thecapacitance of capacitor C_(x). As discussed above, C_(x)═C₁(R_(BC)/R_(AC)). The data stored in memory 52 is searched for thecapacitance of capacitor C_(x) to obtain the corresponding relativeconcentrations. A linear relationship between concentration andcapacitance allows one to normalize any measurement made so as toprovide the absolute concentration of each component in the solution. Ifthe capacitance of capacitor C_(x) is not found in the pre-stored data,the stored data may be interpolated or extrapolated to obtain aconcentration corresponding to the measure capacitance of capacitorC_(x). As noted above, frequency of the waveform generated by voltagesource 22 will influence the response of capacitors. Where thecapacitance of capacitor C_(x) does not exhibit a suitable linearresponse, an expanded set of data points should be stored in memory 52,so that interpolation or extrapolation is unnecessary.

[0045] It should be appreciated that while a preferred embodiment of thepresent invention uses a measure of a capacitor's capacitance todetermine relative concentrations, it is also contemplated that ameasure of other electrical properties of a capacitor may be used todetermine relative concentrations, including, but not limited to, thepermittivity and dielectric constant of the capacitor dielectric.

[0046] Based upon the determined relative concentrations, processingunit 50 may be programmed to control the concentration of one or morecomponents of the decontamination solution. For instance, processingunit 50 may output control signals (see FIG. 1) to adjust a flow controlvalve or other control means for modifying the relative concentrations.Accordingly, processing unit 50 may provide feedback control to adjustthe relative concentrations to correspond with desired relativeconcentrations that provide optimum decontamination. Processing unit 50may also output signals to output unit 60 to provide an audible and/orvisual indicator when the determined relative concentrations are notwithin a desired range. The visual indicator may assist an operator byincluding a display of the relative concentrations or absoluteconcentration of an oxidant or sterilant as determined by processingunit 50.

[0047] In a preferred embodiment, the multi-component decontaminationsolution is comprised of two components, namely, an antimicrobialchemical and a base fluid. The antimicrobial chemical is the activechemical for a decontamination process, while the base fluid acts as adiluent for the antimicrobial chemical, or as a vehicle or carrier forthe antimicrobial chemical.

[0048] Examples of antimicrobial chemicals, include, but are not limitedto, liquids, such as hydrogen peroxide, peracids such as peracetic acid,and bleach, as well as gases, such as ozone, ammonia, ethylene oxide,fluorine containing chemicals, chlorine containing chemicals, and otherhighly oxidative gases.

[0049] Examples of base fluids, include, but are not limited to, water,deionized water, distilled water, an alcohol (e.g., a tertiary alcohol),a glycol-containing chemical compound, and a mixture thereof.Glycol-containing chemical compounds include, but are not limited to,polyethylene glycol, diethylene glycol, triethylene glycol,tetraethylene glycol, glycol ethers, polypropylene glycol, propyleneglycol, and combinations thereof.

[0050] Some typical combinations of an antimicrobial chemical and a basefluid, include, but are not limited to, hydrogen peroxide and water,bleach and water, ozone and water, peracid and water, peracetic acid andwater, alcohol and water, and ozone dissolved in a glycol, or analcohol, such as a tertiary alcohol.

[0051] It is contemplated that the present invention may also besuitably used in a decontamination process to determine whether rinsewater during a decontamination process “rinse cycle” is devoid of anantimicrobial chemical. In this regard, measured capacitance can be usedto assure that no measurable, residual, antimicrobial chemical is to befound in the rinse water. Furthermore, any other chemicals that arepresent in measurable concentrations would be indicated by the measuredcapacitance. Visual and/or audible signals may alert the operator thatobjects (e.g., medical instruments) undergoing a decontamination processmay not be clean or sterile.

[0052] In an alternative embodiment of the present invention, two sensorcircuits 20 are used. Capacitor C_(x1) of the first sensor circuit 20 isexposed to a chemical solution comprised of first and second chemicalcomponents (e.g., a solution of an antimicrobial chemical and a basefluid). Capacitor C_(x2) of the second sensor circuit 20 is exposed onlyto the second component of the two-component chemical solution (e.g.,the base fluid). Processing unit 50 calculates the difference betweenthe two measured capacitances C_(x1) and C_(x2) to determine theconcentration of the first chemical component of the solution. In thisregard, the difference in capacitances C_(x1) and C_(x2) will beattributable to the concentration of the first chemical component.

[0053] Other modifications and alterations will occur to others upontheir reading and understanding of the specification. It is intendedthat all such modifications and alterations be included insofar as theycome within the scope of the invention as claimed or the equivalentsthereof.

Having described the invention, the following is claimed:
 1. A chemicalconcentration detecting system for determining a concentration of afirst component in a multi-component solution, comprising: a capacitorhaving first and second plates exposed to the solution, said solutionbeing a dielectric therebetween; and processing means for determining achange in an electrical property of the capacitor, said change in theelectrical property varying according to the concentration of the firstcomponent in the solution.
 2. A chemical concentration detecting systemaccording to claim 1, wherein said first capacitor is selected from thegroup consisting of: a parallel plate capacitor, a cylindricalcapacitor, and a spherical capacitor.
 3. A chemical concentrationdetecting system according to claim 1, wherein said processing meansincludes a memory for storing a set of data including capacitance valuesand corresponding concentration values indicative of the relativeconcentration of the first component in the solution.
 4. A chemicalconcentration detecting system according to claim 3, wherein saidprocessing means obtains a relative concentration from said set of data,in accordance with said first capacitance.
 5. A chemical concentrationdetecting system according to claim 3, wherein said processing meansuses said set of data to interpolate or extrapolate a relativeconcentration corresponding to the first capacitance.
 6. A chemicalconcentration detecting system according to claim 3, wherein saidprocessing means normalizes said relative concentration to provide anabsolute concentration of the first component of the solution.
 7. Achemical concentration detecting system according to claim 1, whereinsaid multi-component solution is comprised of an antimicrobial chemicaland a base fluid.
 8. A chemical concentration detecting system accordingto claim 7, wherein said base fluid is at least one of: (a) a diluentfor the antimicrobial chemical, and (b) a vehicle for the antimicrobialchemical.
 9. A chemical concentration detecting system according toclaim 7, wherein said antimicrobial chemical is selected from a groupconsisting of: a liquid and an oxidative gas.
 10. A chemicalconcentration detecting system according to claim 9, wherein saidantimicrobial chemical is selected from a group consisting of: hydrogenperoxide, peracid, peracetic acid, bleach, ozone, ammonia, ethyleneoxide, fluorine containing chemicals, and chlorine containing chemicals.11. A chemical concentration detecting system according to claim 7,wherein said base fluid is selected from a group consisting of: water,an alcohol, a glycol-containing chemical compound, and combinationsthereof.
 12. A chemical concentration detecting system according toclaim 11, wherein said glycol-containing chemical compound is selectedfrom a group consisting of: polyethylene glycol, diethylene glycol,triethylene glycol, tetraethylene glycol, glycol ethers, polypropyleneglycol, propylene glycol, and combinations thereof.
 13. A chemicalconcentration detecting system according to claim 11, wherein saidalcohol is a tertiary alcohol.
 14. A chemical concentration detectingsystem according to claim 1, wherein said system further comprises: abridge circuit, wherein said capacitor forms a part of said bridgecircuit.
 15. A chemical concentration detecting system according toclaim 14, wherein said bridge circuit includes a potentiometer havingfirst and second resistances associated therewith, said processing meansdetermining values of said first and second resistances when the bridgecircuit is in a null condition to determine capacitance of saidcapacitor.
 16. A method for determining a concentration of a firstcomponent in a multi-component chemical solution, comprising: exposing acapacitor, having first and second parallel plates, to the solution,said solution comprising a dielectric therebetween; and determining achange in an electrical property of the capacitor, said change in theelectrical property varying according to the concentration of the firstcomponent in the solution.
 17. A method according to claim 16, whereinsaid step of determining the change in an electrical property of thecapacitor includes: accessing pre-stored data including capacitancevalues and corresponding concentration values indicative of theconcentration of the first component in the solution.
 18. A methodaccording to claim 17, wherein said step of determining the change inthe electrical property of the capacitor includes: interpolating orextrapolating from the pre-stored data the concentration of the firstcomponent in the solution.
 19. A method according to claim 16, whereinsaid processing means normalizes said concentration to provide anabsolute concentration of the first component of said solution.
 20. Amethod according to claim 16, wherein said solution is comprised of anantimicrobial chemical and a base fluid.
 21. A method according to claim20, wherein said base fluid is at least one of: (a) a diluent for theantimicrobial chemical, and (b) a vehicle for the antimicrobialchemical.
 22. A method according to claim 20, wherein said antimicrobialchemical is selected from a group consisting of: a liquid and anoxidative gas.
 23. A method according to claim 20, wherein saidantimicrobial chemical is selected from a group consisting of: hydrogenperoxide, peracid, peracetic acid, bleach, ozone, ammonia, ethyleneoxide, fluorine containing chemicals, and chlorine containing chemicals.24. A method according to claim 20, wherein said base fluid is selectedfrom a group consisting of: water, an alcohol, a glycol-containingchemical compound, and combinations thereof.
 25. A method according toclaim 24, wherein said glycol-containing chemical compound is selectedfrom a group consisting of: polyethylene glycol, diethylene glycol,triethylene glycol, tetraethylene glycol, glycol ethers, polypropyleneglycol, propylene glycol, and combinations thereof.
 26. A methodaccording to claim 24, wherein said alcohol is a tertiary alcohol.